Heretofore, as a flush water tank assembly for supplying flush water to a toilet main unit, there has been known one type which comprises a small tank formed in a tub-like shape having a bottom wall with an opening, and a float provided inside the small tank, wherein the float is adapted, when flush water held in the small tank is drained from the opening with an openable and closable valve (check valve), to be lowered along with a lowering of a water level within the small tank, causing a water supply valve to be opened interlockingly with the float, as described, for example, in the Patent Document 1.
In this type of conventional flush water supply device, in order to reduce an amount of flush water that is supplied, when the water supply valve is opened, into the flush water tank and discharged to the toilet main unit together with flush water preliminarily stored in the flush water tank (so-called “tail flushing water”) during the time after a water discharge valve is opened and a water level inside the flush water tank starts lowering until the water discharge valve is closed, a valve opening time adjuster is provided which is capable of adjusting a water supply valve opening time for supplying flush water into the tank by opening the water supply valve, thereby to cause the time when the water supply valve is opened to be delayed.